Cardiovascular diseases, such as acute myocardial infarction, stroke, peripheral arterial occlusion, pulmonary embolisms, deep vein thrombosis, and other blood vessel thrombotic diseases are major causes of morbidity and mortality. The aforementioned diseases are caused by total or subtotal occlusive thrombus formation in a blood vessel, which prevents delivery of an adequate blood supply to the tissue. The thrombus consists of aggregates of blood cells such as platelets, erythrocytes, and leukocytes, stabilized by a fibrin network.
Current therapeutic approaches to these thrombotic vascular diseases involves lysis of the existing thrombus and prevention of recurrent thrombus formation, leading to reocclusion of the formerly reopened vessel.
Thrombolytic therapy of acute myocardial infarction has been shown to markedly improve the natural history of acute myocardial infarction, with an approximately 30% reduction in mortality (GISSI: Lancet 1986; 1: 871-874; ISIS-2: Lancet 1988; 2: 349-360; AIMS: Lancet 1988; 1: 545-549; Wilcox et al., Lancet 1988: 2: 525-539; ISAM: N Engl J Med 1986; 314: 1465-1471). The findings of the recently completed GUSTO-trial (Global Utilisation of Streptokinase and Tissue-type plasminogen activator for Occluded coronary arteries) indicated that accelerated t-PA given with intravenous (i.v.) heparin provided a survival benefit over previous standard thrombolytic regimens (GUSTO: N Engl J Med 1993; 329: 673-682). More importantly, the study supported the hypothesis that more rapid and complete restoration of coronary blood flow through the infarct-related artery resulted in improved ventricular performance and lower mortality among patients with myocardial infarction. (GUSTO; N Engl J Med 1993; 329: 1615-1622).
However, recent data suggest that current reperfusion strategies do not realize the maximum potential for reduction of mortality and salvage of ventricular function (Lincoff and Topol, Circulation 1993; 87: 1792-1805). The benefits of thrombolysis substantially deteriorate in many patients due to insufficiently early or rapid recanalization, incomplete patency with TIMI grade 3 flow or critical residual stenosis, absence of myocardial tissue reflow despite epicardial artery patency, intermittent coronary patency, subsequent reocclusion, or reperfusion injury. Therefore, there are efforts underway to achieve optimal reperfusion. These efforts are directed, for the most part, at enhancement of the velocity and quality of thrombolysis.
Pharmacological approaches to enhancing velocity and quality of thrombolysis can, in general, be based upon the thrombolytic agent itself and upon adjunctive agents, i.e., other agents given concomitantly to the thrombolytic agent.
Recombinant tissue-type plasminogen activator (rt-PA) has been shown to achieve higher patency rates resulting in lower mortality when a total dose of 100 mg was administered in an accelerated regimen, i.e., within 90 min, instead in the conventional, approved 3-h regimen (ISIS-3: Lancet 1992: 339: 753-770; GUSTO: N Engl J Med 1993; 329: 673-682). Apart from modifying the administration regimen of rt-PA, the use of a novel thrombolytically active protein, such as the novel recombinant plasminogen activator BM 06.022, described in U.S. Pat. No. 5,223,256 and incorporated by reference was shown to achieve very high patency rates after double bolus administration (Bode et al., Circulation 1993; 88 (suppl. I): I-292, abstract 1562).
The problem of reocclusion of the infarct-related artery after successful reperfusion has been recognized to be associated with substantial morbidity and mortality rates (Ohman et al., Circulation 1990; 82: 781-791). Therefore, pharmacologic strategies aim at reducing reocclusion and sustaining infarcted artery patency. Since activation of platelets and of the coagulation system after administration of thrombolytic agents has been shown to be involved, for the most part, in the pathogenesis of reocclusion, attempts are being made to pharmacologically inhibit platelet aggregation and coagulation. Therefore, the use of aspirin, an antiplatelet agent, and of heparin, an anticoagulant agent, is usually recommended in combination with the thrombolytic agents when treating acute myocardial infarction (Popma and Topol, Ann Int Med 1991; 115: 34-44).
However, the efficacies of aspirin and heparin are limited. This is attributable to their modes of action. Aspirin only inhibits one pathway of activation of platelets (by inhibition of cyclooxygenase). The action of heparin is dependent on the availability of antithrombin III. The restricted efficacy of heparin is also caused by the presence of inhibitors in plasma and its limited access to clot-bound thrombin. Therefore, there is a great deal of interest in novel antiplatelet agents, such as antagonists of the glycoprotein IIb/IIIa receptor (e.g., antibodies, peptides, or low molecular weight chemical entities) and in novel anticoagulants (peptidic and synthetic direct inhibitors of thrombin and other components of the coagulation system, such as inhibitors of factor Xa, IXa, VIIa, tissue factor, etc., or mimics of endogenous inhibitors of the coagulation system, such as activated protein C or thrombomodulin).
Recently, clinical trials began evaluating the usefulness of combining t-PA with the chimeric 7E3 antibody, which binds to the glycoprotein IIb/IIIa receptor (Kleiman et al., J Am Coll Cardiol 1993; 22: 381-389). Several clinical trials have already been performed to study the effect of combination of a thrombolytic agent with novel direct inhibitors of the coagulation system. The combination of accelerated t-PA and hirudin (a recombinant protein which directly inhibits clot-bound thrombin) resulted in prevention of reocclusion and high TIMI grade III patency rates (Cannon et al., J Am Coll Cardiol 1993; 21: 136A and Neuhaus et al., Circulation 1993; 88 (suppl. I): 1-292, abstract 1563). The hirudin like peptide hirulog was combined with infusion of streptokinase; the study showed that clot lysis occurred more rapidly after streptokinase plus hirulog (Lidon et al., J Am Coll Cardiol 1993; 21: 419A).
Since the use of t-PA is associated with a high reocclusion rate (10-20%) after thrombolysis despite the use of aspirin and heparin (Neuhaus et al., J Am Coll Cardiol 1989; 14: 1566-1569; Cheseboro et al., Circulation 1987; 76: 142-154; Neuhaus et al., J Am Coll Cardiol 1988; 12: 581-587; Califf et al., Circulation 1991; 83: 1543-1556; Neuhaus et al., J Am Coll Cardiol 1992; 19: 885-891), the administration of the novel adjunctive agents hirudin and hirulog required prolonged infusion for 36, 48, or 96 h. This long infusion period means that huge amounts of recombinant protein (=hirudin) or synthetic peptide (=hirulog) are required: 532 or 546 mg of hirudin or 1008 mg of hirulog as calculated by multiplying the dose (mg/kg/h) with the mean body weight of a human (70 kg) and with the duration of infusion described in the above mentioned abstracts. Large amounts of protein or synthetic peptide are expensive which in turn leads to high costs and is medically disadvantageous, since the high price of the anticoagulant prevents widespread use. Furthermore, the administration of an infusion increases costs, since the technique requires infusion machines, monitoring of the anticoagulant effectiveness, and medical staff to control the infusion. These obstacles limit the broad use and application of hirudin and, thereby, many patients will not profit from its benefits.
The combined results of the materials GISSI-2 (Lancet 1990; 336: 65-71) and ISIS-2 (Lancet 1988; 2: 349-360) demonstrated a "significantly increased incidence of cerebral hemorrhage and major noncerebral bleeds with the addition of heparin to the thrombolytic/aspirin regimen" which is medically very disadvantageous (Lincoff and Topol, Circulation 1993; 87: 1792-1805). Since early clinical experience with hirudin indicated that spontaneous hemorrhaging occurred after administration of hirudin, t-PA, and aspirin and that there was an increase in catheter site bleeding (Neuhaus et al., Circulation 1993; 88 (suppl. I): I-292, abstract 1563), there seems to be no reduction of the bleeding risk when replacing heparin-infusion by hirudin-infusion.
Experimental evaluation of the effect of combining hirudin with a thrombolytically active protein has always been performed with infusion of hirudin. Exemplary are reports on t-PA plus hirudin (Haskel et al., Circulation 1991; 83: 1048-1056), streptokinase plus hirudin (Rigel et al., Circ Res 1993; 72: 1091-1102) and BM 06.022 SEQ ID NO: 1 plus hirudin (Martin et al., Int J Hematol 1992; 56: 143-153). All these experimental studies have shown that hirudin-infusion was superior to heparin-infusion in improving coronary blood flow after reperfusion. Administration of heparin as a single i.v. bolus injection plus the thrombolytic agent BM 06.022 SEQ ID NO: 1 was not superior to heparin-infusion plus BM 06.022 SEQ ID NO: 1 (Martin et al., J Am Coll Cardiol 1993; 22: 914-920).
Clinical experience has shown that reocclusion occurred from after reperfusion up to hospital discharge of the patient at 7-21 days after reperfusion, with a peak incidence within the first few days after thrombolysis (Ohman et al., Circulation 1990; 82: 781-791). This observation explains why potent anticoagulation has to be prolonged for several days after thrombolytic treatment.
Pharmacokinetic analysis showed that hirudin has a short half life of 10-15 min in dogs (Biomed Biochim Acta 1987; 46: 237-244 and Folia Haematol 1988; 115: 70-74) and of 9-50 min in humans (Thromb Haemost 1984; 52: 160-163).
The experimental, clinical, and pharmacokinetic data discussed supra suggest that it is necessary to administer anticoagulant by continuous i.v. infusion in order to achieve reliably adequate plasma levels for anticoagulation in the treatment of acute vascular diseases. Current clinical evaluation of hirudin as a novel anticoagulant in combination with thrombolytically active proteins follows this line of reasoning.
Notwithstanding these suggestions, there still exists a need to reduce the amount of protein, peptide, or chemical entity used in thrombolytic treatment, a need to simplify administration, and a need to reduce bleeding risk by restricting the potency and anticoagulant efficacy of a drug to an optimum. This optimum combines a desire for the maximum duration of the effect of improving coronary blood flow quality and the minimum duration of unwanted side effects such as minor and major bleeding events and intracerebral hemorrhage.